The present invention relates to an extrusion apparatus for extruding thermoplastic material. In particular, the present invention relates to a barrel and screw extrusion apparatus for extruding solid plugs of polymer resin.
As disclosed for example in the following patents, barrel and screw type extrusion apparatus are well known, and specific embodiments thereof have been provided for solving various material extrusion problems, or for meeting various material extrusion objectives.
In U.S. Pat. No. 3,461,497 issued Aug. 19, 1969 to J. W. Geyer, a barrel and screw extrusion apparatus is disclosed for handling a wide variety of extrudable materials. The disclosed apparatus includes a screw that has generally saw-tooth shaped threads which face in the direction of material advance.
To optimize the mixing of material being extruded, U.S. Pat. No. 3,946,998 issued Mar. 30, 1976 to Menges et al, discloses an extrusion apparatus that includes an improved screw. The improved screw has partial screw thread sections for irregularly dividing and thereby mixing material being extruded.
In order to reduce stress and overheating, U.S. Pat. No. 4,408,888 issued Oct. 11, 1983 to Hanslik discloses an extrusion apparatus that includes conically shaped, double-worm screws.
U.S. Pat. No. 4,215,978 which issued Aug. 5, 1980 to Takayama et al discloses a resin molding screw that improves blending and melting of resin material. The screw includes a minor diameter surface that has wavy contoured portions and barriers.
To minimize pressure which opposes material flow, U.S. Pat. No. 4,642,041 issued Feb. 10, 1987 to Murphy discloses an extruder screw that includes a feed section, a transition section, a metering section, and a plurality of back flow seals.
U.S. Pat. No. 5,020,915 issued Jun. 4, 1991 to Julien discloses an extrusion screw that includes a helical thread having a trapezoidal profile, a flat thread crest and inclined flanks for increasing flight width. Increasing the flight width increases compression of the material being extruded, thereby resulting in proper material homogeneity.
Despite the various improvements represented by the above examples; an almost universal objective that remains to be accomplished is increasing the rate and stability of extruded material flow. In this respect, it has been found by the applicants for example that in a barrel and screw extruder, in order to increase material feed pressure as well as increase the rate and stability of the flow or throughput of material being extruded, the stress between the material and the barrel surface should be made greater than the stress between such material and the minor circumferential surface of the screw. In other words, the force between material, for example a solid resin feed plug, and the barrel, should be greater than the force between such material and the extrusion screw. In fact, the greater the difference between these two forces, the greater will be the feed pressure, the material flow rate and flow stability.
There is therefore a need for a relatively inexpensive extrusion apparatus that attempts to achieve such a maximization of this difference between these extrusion forces, and to do so without fouling subsequent batches of material being extruded.